The Type III secretion (T3S) system is a multicomponent transport apparatus that mediates the assembly of both the bacterial flagellum and the virulence-associated injectisomes used by a wide variety of pathogens. Secretion is driven by proton motive force. FlhE is a member of the flagellar T3S system, whose absence causes a proton leak in E. coli and Salmonella, concomitant with cell death. In all T3S systems, there is a set of six conserved integral membrane (IM) proteins essential for secretion. Despite this conservation, it is not known which of these proteins actually constitute the secretion pore, and which play only supportive roles. This proposal seeks to exploit the properties of FlhE in defining the secretion pore. In the E. coli/Salmonella flagellar system, FlhA-FlhB are speculated to comprise the 'export gate'. FlhE is transcribed as part of the flhBAE operon, hence likely plays a role in FlhAB function. FlhE is a periplasmic protein, included within the flagellar basal body lumen. We isolated FlhE from the periplasm and solved its structure. The 1.5 A resolution structure suggests that FlhE may serve as a plug for the secretion pore. With the FlhE structure as guide, we propose to test how FlhE acts as a plug. These experiments will identify the constituents of the secretion channel and its supporting scaffold. The similar overall architecture of the basal structure of flagella and injectisomes, conservation of several of their T3S component proteins, as well as similarities in their PMF-driven secretion mechanism, suggests that lessons obtained from one system are transferable to the other. Because absence of FlhE causes substantial cell death, the proposed studies could potentially suggest rational drug-design strategies for targeting enteric pathogens where FlhE is mainly found. In the long-term, we anticipate that the knowledge gained from these studies could be used to target PMF-driven multidrug efflux pumps or other essential proton-driven secretion channels.